I'm not sure what the Cortaid coupon said but if it didn't forbid overage, you should have got the overage if the coupon scanned.
DG allows overage to be applied to other items (excluding giftcards), but you can't get cash back. A long as the coupon will scan, you get the overage. They can't manually enter a coupon for more than the item cost but if the coupon scans, it will allow overage.
Thanks for posting about this.